Coating treatment of metal surface

ABSTRACT

IMPROVED CORROSION RESISTANT FILMS ARE FORMED ON METAL SURFACES AND COATING PROPERTIES ARE IMPROVED BY IMMERSING METALS IN SOLUTIONS OF MOLYBDIC ACID OR SALT THEREOF AND A WATER SOLUBLE ORGANIC HIGH POLYMER. THE TREATMENT IS PARTICULARLY EFFECTIVE FOR STEEL AND GALVANIZED STEEL SHEETS AND PIPES.

i d S emen US. Cl. 148-614 7 Claims ABSTRACT or DISCLOSURE Improved corrosion resistant films are formed on metal surfaces and coating properties are improved by immersing metals in solutions of molybdic acid or salt thereof and a water soluble organic high polymer. The treatment is particularly elfective for steel and galvanized steel sheets and pipes.

This invention relates to the coating treatment of metal surfaces, especially of steel and galvanized steel sheets and pipes, to improve the corrosion resistance and coating property by immersing the metals in a solution or emulsion containing molybdate thereof and a water-soluble organic high polymer, and then by heating the treated surface in order to form a film thereon.

It is well known in the prior art that chromic acid and phosphate treatments have been conducted in order to improve the corrosion resistance and coating property of steel sheets and galvanized steel sheets. However, conventional chromic acid and phosphate treatments have not been satisfactory with regard to corrosion resistance; the chromic acid treatment has been particularly unsatisfactory. Each of such treatments have resulted in inferior coating property. To obviate such deficiencies, various modifications have been proposed. Up to date, only unsatisfactory industrial results have been obtained.

It is accordingly a primary object of the present invention to obviate the above-mentioned.difliculties and to .25

and the polymer or polymers cooperate to form an excel-= lent insoluble film on the metal surfaces.

Molybdenum-containing compounds used herein include molybdic acid or a salt thereof. Preferred salts are ammonium, sodium and calcium molybdate. Extremely small quantities of one or more of the acid and salts cant-i be used. The solutions can contain from about 0.1 to about 30 percent by weight of such molybdenum compounds. Concentrations greater than about 0.5 mol of such compound in the solution of molybdenum compound and polymer are not advantageous.

The polymers of this invention are water-soluble organic polymers of high molecular weight, greater than about 10,000 (Viscosity Method). Such polymers include: acrylic, methacrylic, butadiene, styrene, maleic, linseed oil, polyvinyl alcohol, polyvinyl ether, alkyd, melam'inealkyd, melamine-formaldehyde and copolymers thereof. The concentration of resin in the treating solution ranges between about 0.05 and about percent by weight.

When high level corrosion resistance is desired, the concentration of the treating solution is increased. The amount of coating on the metal surface is also increased.

3,586,543 Patented June 22, 1971 With such increased concentrations and coatings, the heating or baking conditions are also varied in accordance .with the type and amount of polymer used. Generally,

higher baking temperatures are employed.

The treating solutions preferably contain an acid or salt thereof. Such additives can be used to adjust the pH of the solutions. Typical additives are mineral acids such as surfuric and phosphoric, organic acids such as acetic acid, acrylic acid, tartaric acid and citric acid, and salts such as sodium salt, potassium salt and calcium salt can be used.

Metal surfaces which have been immersed in the treating solutions are scorched or heated. Generally, they are subjected to temperatures of from about 200 C. to about 450 C. for intervals of from about /5 to about 2 minutes.

The invention is illustrated, and in no sense, limited by the following examples,

EXAMPLE 1 A treating solution of 70 parts of a melamine alkyd resin 5% solution, 15 parts sodium molybdate 0.1 N solution, and 0.5 part of phosphoric acid 10% solution is used. A cold rolled steel sheet is immersed in the treating solution, removed therefrom and passed through a neoprene rubber grooved roll for even distribution. It is then baked. The treated coated steel sheet has a yellowish, half-polish appearance. It was subjected to salt water spray (HS) and outdoor exposure tests, the results of which are about the same as those obtained in the same tests given to marketed steel sheets coated with electrolytic chromic acid. In a wetting test (49 C., 98% humidity), while marketed steel sheets coated by electrolytic chromic acid showed 50% of the area rusted after only 6 hours,

those treated with the method according to this invention showed the same degree of rust after 12 to 24 hours. As to the coating property of melamine alkyd and acrylic coating materials, the method used in the present invention showed the same degree of superior values as those given to cold rolled steel sheets in various tests applied on coated surfaces, in crosscut, bending, drawing, impact and Ericsen tests.

EXAMPLE 2 Cold rolled steel sheet is immersed in a treating solution of 10 parts of magnesium molybdate of 0.1 N solution, and 70 parts of 7% melamine alyd resin, the sheet is then taken out of the treating solution and is baked. Corrosion resistance and coating property are tested in the same manner as in Example 1 and similarly superior results have been obtained.

EXAMPLE 3 Cold rolled steel sheet is immersed in a treating solution of 70 parts of 4/ alkyd (phthalic acid) resin solution, 15 parts of sodium molybdate 0.1 N solution and 1 part of 10% sulfuric acid. Itis then taken out of the treating solution and is baked. Corrosion resistance and coating property have been tested in the same manner as inthe case of Example 1 andthe same superior results have been obtained.

EXAMPLE 4 invention. 7

3 EXAMPLE Galvanized steel sheet is treated'in a treating soiution" of 70 parts of 4% melamine alkyd resin solution, 5 parts of sodium molybdate of 0.1 N solution, and, 0.3 part of 510% sulfuric acid. The sheet is removed from the treating solution and is passed through a roll. It is scorched by *heat. The result of the Wetting test (49 C., 98% humidity) with the treatedsheet, indicated improvement in corf rosion resistance toward white rust. Coating property when tested in the same manner as in the case ofExample lturned out to be just as excellent as in the case of coatings given to cold rolled steel sheets by the method of this EXAMPLE 6 according to this invention is applied to cold rolled i steel sheet, it is possible to obtain a product with sufiicient corrosion resistance required before being put to second process coating; and the same degree of superior coating When this treatment is applied to galvanized steel sheets, the sheets have highly superior corrosion resistance toward white rust as compared with galvanized steel sheets subjected to a prior art chrome acid treatment. Further, galvanized steel sheets treated according to this invention have the same degree of superior coating property as in the case of cold rolled steel sheets.

Besides superior corrosion resistance and coating prop- .erty, improvement in workability is to be expected. A

superior efifect in after-treatment of coated steel sheets other than galvanized steel sheets is also obtained. A

Additional test results are provided in the following tables wherein:

Table I shows the results of JISZ 22371 salt water spray tests, that have been employed in Example 1. Steel sheets coated with 2 g./m. of resin according to the method of this invention show a far more remarkable corrosion 45 resistance thansteel sheets treated by the electrolytic ohromic acid and available in the market.

Table II shows results of wetting tests (49 C., 98% humidity). Galvanized steel sheets coated with 0.5 g./m.

resiri'according to the method of this invention show a far more excellent corrosion resistance against white rust than those treated by prior art chromic acid treatment.

TABLE I.-JIS SALT WATER SPRAY TEST 1 Test time (hours) 1 3 5 p s Marketed steel sheet treated with electrolytic chromic acid:

p 50 100 c 100 Steel sheet treated according to t vention (examples 1, 2, 3, 4) 10 25 100 1 Values indicate percentage of rusted area.

TABLE II.WETTING TEST 1 Test time (hours) 1 2 5 10 24 Galvanized steel sheet treated with chromic acid 50 100 Galvanized steel sheet treated with the method according to the present invention (examples 5, 6) None None 10 50 values'indieate percentage of rusted area.

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Resin solutions employed in the Examples to 6 are identifiedebelow for abetter understanding of the invention.

Alyd (phthalic acid) resin is a denaturation of a phthalic alkyd resin being condensated with phthalic anhydrate W 0 CO A 7 CHZQH O and glycerlne CHOH 'CH OH and then denaturated with linseed Loil (oil content of 7 about 50%60%) Melamine alkyd resin is a blending of denaturation of a melamine which have been denaturated with coconut oil and then condensated with said phthalic alkyd resin and coconut oil and then the melamine resin being condensated with said melamine and formaline (HCH-O) is blended.

Melamine alkyd resin 5% solution used in the Example 1 is identified here as 5% aqueous solution of melamine alkyd resin. Two resins identified hereinabove have been used in the examples.

What is claimed is: v 1. The method of coating a steel surface to improve the corrosion resistance and coating property thereof,

which comprises:

immersing the steel in an aqueous solution containing from about 0.1 to about 30 percent by weight of molybdic acid or salt thereof and from about 0.05 to about 45 percent by weight of a water-soluble organic polymer having a molecular Weight of greati er than about 10,000 and heating the steel.

2. The method of claim 1 wherein the metal is galvanized steel.

3. The method of claim 1 wherein the salt is an ammonium, sodium or calcium molybdate.

4. The method of claim 1 wherein the polymer is selected from the group consisting of acrylic, methacrylic,

butadiene', styrene, maleic acid, linseed oil, polyvinyl alcohol, polyvinyl ether, alkyd, melamine-alkyd, melamineformaldehyde polymers and copolymers thereof.

5. The method of claim' 1 wherein the solution contains a mineral acid or an organic acid to adjust the pH of said solution.

6. The method of claim 1, wherein the steel which has been so immersed is rolled before being heated.

7. The method of claim 1, wherein the steel is heated at a temperature of from about 200 C. to about 450 C.

References Cited UNITED STATES PATENTS 3,052,575 9/1962 Crane 148-6.14 3,175,964 3/1965 Watanabe et al. 148--6.2X 3,185,596 5/1965 Schifiman 1486.2X

RALPH s. KENDALL, Primary Examiner C. WESTON, Assistant Examiner US. Cl. X.R.

PMOYO UNITED ST ATES PATENT OFFICE CEHTHRCATL Oh CORRELLE ION Patent No. 3 586, 543 Dated June 22, 1971 Inventor) HIROSHI IIJIMA et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Assignee is incorrectly identified should be Nippon Kgkan 'Kabushiki Kaisha, as indicated in Assignment recorded in Patent Office on October 16,

1968 on Reel 2424 Frame 145.

Column 2, line 47 delete "alyd" and replace with --alkyd--- as indicated in specification, page 6,

line 11.

Column 2, line 55 delete "4/" and replace with 4%-- as indicated in specification at page 6,

line 1?.

Column 3, lines 6-7 delete "scorched by heat" and replace with -baked--- as indicated in amendment filed September 23, 1970 at page 3 (re page 7, line 17 I- of specification) UNITED STATES PATENT OFFICE PO-iOiiO CERTIFICATE QT CGRREQTTON Patent NO. 3,586, Dated June 22, 197].

Inventor(s) HIR-QSHI IIJIMA et al 2 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 27 after "coating" (second occurrence) insert ---property is obtained as in the case of cold rolled sheets.-- as indicated in specification at page 8, lines 16-17.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Attesting Officer ROBERT GOITSCHALK Acting Commissioner of Patents 

